Known automatic or automated motor vehicle transmissions, as a rule, comprise a parking lock by which a drive output shaft of the transmission can be mechanically locked. Besides a purely mechanical system for actuating a parking lock of that type by means of an active mechanical connection between a selector lever that can be operated by the driver in the interior space of the motor vehicle and the parking lock itself in the transmission, electro-mechanical and electro-hydraulic systems for actuating such a parking lock are also known—for example from DE 4127991 C2—, in which the parking lock in the transmission is connected, via an active electrical connection, to a selector device that can be operated by the driver inside the motor vehicle, and in which the parking lock can be actuated electro-mechanically or electro-hydraulically by a parking lock function implemented in an electronic control unit of the transmission as a function of a shift position selected with the selector device and as a function of other operating parameters of the motor vehicle.
Moreover various methods for the automatic engagement of such a parking lock are known. For example, in U.S. Pat. No. 3,937,105 it is proposed to engage the parking lock of the transmission automatically, independently of the shift position selected by the driver if an ignition current circuit of the motor vehicle is interrupted and at the same time the motor vehicle is still rolling at a speed lower than a defined low threshold value.
From U.S. Pat. No. 4,892,014 a method is known, in which the parking lock of the transmission is engaged automatically by an electric motor, on the one hand if an ignition current circuit of the motor vehicle is interrupted and at the same time the vehicle's speed is lower than a defined low threshold value, and on the other hand also if the driver's door of the motor vehicle is open and the driving seat of the motor vehicle is unoccupied and at the same time the speed of the vehicle is less than the low threshold value. In both cases, when the parking lock is automatically engaged a parking brake of the motor vehicle is also actuated automatically. In U.S. Pat. No. 4,892,014 it is also proposed that when the ignition current circuit is closed but the drive engine of the motor vehicle is not running, the transmission is automatically shifted by an electric motor to its neutral position without this having been called for by the driver.
Finally, from EP 0 814 287 B1 a function for the automatic engagement of the parking lock for an automatic transmission is known, in which the parking lock in the automatic transmission is engaged automatically by the transmission control unit, if the driving speed of the motor vehicle is zero and, an ignition current circuit has been interrupted and at the same time, as a further condition, either a predetermined time interval has lapsed since the ignition current circuit was interrupted or a vehicle door is open, but at the latest—and this as the sole condition—if the ignition key has been removed from the ignition lock. To give better vehicle availability, as a special feature in all three cases the engagement of the parking lock is prevented, if the driver has selected the neutral shift position using the selector lever either immediately before, or within a predetermined time interval after the engine is switched off.